A conventional real-time spectrum analyzer generally receives a time domain input signal and displays corresponding frequency domain spectra, where each frequency domain spectrum represents a corresponding time interval. The spectra may be obtained by performing fast Fourier transforms (FFTs) on digital time domain data representing the time domain input signal. The display may be a density histogram display, which visually indicates the density of various frequencies. The density histogram display is periodically updated, where each update may be referred to as a “slice.” Approximately 10,000 FFTs from multiple spectra form each slice.
A frequency mask trigger may be included in a conventional real-time spectrum analyzer to provide triggered measurements when used with a density histogram display. The frequency mask trigger is configured to detect a trigger condition in the spectra based on the frequency mask. When the trigger condition is detected, a fixed amount of data of the spectra is stored (and displayed), and then the trigger is re-armed. However, real-time gap free operation is lost during the data transfer, display, and re-arming processes. Further, using a conventional frequency mask trigger, all of the FFTs in the spectra of a histogram slice are displayed, regardless of how many of the FFTs actually meet the trigger condition. In other words, when the frequency mask trigger is turned on, if the trigger condition has been met for the slice, all of the FFTs of all of the spectra in that slice are displayed, even if only one FFT of one spectrum meets the trigger condition. This may be particularly problematic when the input signal is moving in and out of the trigger condition within a single slice, since the display of the slice will show FFTs that are both inside and outside the trigger condition. Although this feature may be desirable for batch mode stored data, it is not compatible with a continuous visual display of a gap-free density histogram in which a user wants to see only the domain spectra of the input signal that meet the trigger condition.